The Comets
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Like asteroids, comets are pieces of interplanetary rubble remaining from the 
formation of the planets. The difference is that comets formed in the outermost 
fringes of the solar system, far away from the warmth of the Sun. The solid part 
of a comet is called its nucleus. Anywhere from 2 to 50 km across, a nucleus is 
too small to see even with the largest of telescopes. A comet's nucleus is 
essentially a giant dirty snowball, mostly water and ice, coated with a dark 
dust. 

As a comet moves toward the inner solar system, the dark nucleus undergoes and 
incredible change. The Sun warms it, turning exposed icy surfaces directly into 
a gas. The jets become stronger as the comet nears the Sun, the dark crust 
cracks and exposes more ice. A growing cloud of gas and dust completely hides 
the nucleus. Sunlight excites the gas and it glows with a faint bluish light. A 
thin stream of gas rushing away from the Sun pushes back the comet's gas cloud, 
forming one part of the tail. Dust particles take a slightly different route, 
fanning out along the comet's orbit. They reflect light form the Sun, so the 
comet's dust tail glows with a yellow light. If the comet's path brushes the 
Earth's, the dust particles create a meteor shower when they run into the 
atmosphere. 

Comet Halebopp
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Halley's Comet (shown below) is perhaps the most known in the world. It is 
visible to us without aid of a telescope every 76 years. It was last near Earth 
in 1986 and space probes were sent to take a closer look. One space probe passed 
within 600 km of the comet's nucleus and the photographs it sent back show 
violent jets of gas and dust erupting from a dark potato-shaped nucleus just 15 
km long and 8 km
wide. Haley's Comet will be back in 2061. A comet can only last for several 
thousand circuits of the Sun before its store of energy is exhausted and it 
fades away. 

Halley's Comet in 1986
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Astronomers believe that millions of comets orbit the Sun in an area called the 
Oort Cloud, some thousands of times more distant than Pluto. Only the feeble 
gravity of a passing star is needed to send some of these toward the inner solar 
system. 

